Asa no Hikari ga Matenakute

Kosuke is bullied by classmates every day, and stress accumulates. His older sister Mika sympathizes with him very much as she also faces bullying at school. Parents rarely return from work, so Mika offers her body to brother, and the two drown in sexual pleasures. They dream about the time problems disappear, but in reality things become only worse now aggravated by the sense of immorality of incest.

[From vndbreview]